1. Field of the Invention
The present invention relates to a signal modulation circuit, and particularly relates to a circuit for performing delta sigma modulation.
2. Description of the Related Art
Conventionally, delta sigma modulation (ΔΣ modulation) is used in switching amplifiers or the like. A delta sigma modulator has a subtracter, an integrator, a quantizer, and a quantization error feedback circuit.
FIG. 17 illustrates a basic configuration of a delta sigma modulation circuit. A subtracter 16 calculates a difference between an input signal and a feedback signal, and an integrator 10 integrates the difference signal. An integral signal is quantized by a quantizer 14 and is output as, for example, 1 bit (=binary) signal. The quantization error is fed back via a delay device 12.
JP 2007-312258 A discloses a delta sigma modulation circuit including an integrator group, an adder group, a quantizer, and a pulse width round-up circuit, and further discloses that a signal is converted into a one-bit signal synchronized with a sampling clock so as to be output. In addition, JP 2007-312258 A discloses that a D-type flip-flop is used as the quantizer. JP 2012-527187 W also discloses a delta sigma modulation circuit.
In the configuration shown in FIG. 17, the delay device 12 is provided to a feedback path so that noise shaping is performed. At the same time, however, the configuration has a problem that the delay device 12 in the feedback path cannot correct an output state in real time, or a problem that noise shaping is not performed on distortion/noise components generated in the delay device and the distortion/noise components are directly output.
Further, when the delta sigma modulation circuit is used in a one-bit audio amplifier, pulse width modulation (PWM) and pulse density modulation (PDM) are used as a system for modulating an input signal into a one-bit digital signal. When PDM is used, which is suitable for a case where an input signal is expressed by density or frequency of a pulse, a zero level is inserted at predetermined timing so that a pulse width is maintained, and a level of the input signal needs to be reliably modulated into the frequency of the pulse.